While several states have legalized using marijuana, nobody is in favor of driving under its influence. But as states crack down on high driving, they're struggling to figure out what counts as driving high, and how they can reliably test for it.

Currently, if police determine they have reason to suspect a driver of being under the influence -- perhaps due to erratic driving or other behavior -- they can administer a field sobriety test. But unlike alcohol, there's no on-scene breathalyzer for marijuana, and so in order to confirm marijuana use, officers must take drivers to police stations to administer a blood test.

But even if after all that, officers are able to get that test and prove a driver had marijuana in his or her system, there's a debate over what level really means the driver was actually under the drug's influence.

To combat marijuana-impaired driving, some states have established a clear legal standard for judging whether someone is driving under the influence: 5 nanograms of THC (the primary intoxicant in marijuana) per milliliter of blood.

But new research shows that even substantially impaired marijuana users would not be caught under this standard if they consumed pot in edible form.

Unlike alcohol, which is nearly exclusively consumed in beverage form, marijuana can be consumed via a variety of methods. While it's primarily smoked, as the legalization and commercialization of marijuana has spread, cannabis-infused sodas, tinctures, cookies, and candies have become increasingly popular.

That poses a challenge for regulators: Because edible cannabis is absorbed through the stomach rather than the lungs, most researchers suspect that edible and smoked marijuana affect the body differently. One such researcher, Johns Hopkins University professor Ryan Vandrey, recently studied whether consuming marijuana edibles raises blood levels of THC sufficiently for a stoned driver to be defined as legally impaired.

Participants in Vandrey’s study consumed marijuana brownies of varying strength that matched the levels of THC commonly available in the recreational and medical markets. The study continuously monitored participants’ blood for the presence of THC over several days. Although the marijuana brownies caused significant decrements in participants’ attentiveness, physical coordination and ability to concentrate, only 11 percent of them ever had a THC blood level over the legal limit. In other words, although the participants would have been very high-risk drivers, current law defined 89 percent of them as nonimpaired.

“Our findings tell me that 5 nanograms of THC per milliliter of blood is just not a good marker for intoxication,” Vandrey says. Attorney and highway safety expert Stephen Talpins agrees, arguing that the current standard is “a license to use with near impunity. There should be zero tolerance for people driving with any amount of THC in their blood.”

A zero tolerance standard would solve most of the problems of marijuana edible-impaired drivers going undetected, although a few of Vandrey’s study participants never showed any detectable level of THC in their blood. But a zero-tolerance standard might unfairly convict heavy pot users of impaired driving, because small amounts of THC can linger in their blood for days or even weeks after their most recent toke.

Detecting and deterring marijuana-impaired driving thus remains a significant headache for policymakers and law enforcement. More research like Vandrey’s is clearly needed to guide the development of policy that is both firm and fair, but there’s no guarantee lawmakers will wait for scientific results on the topic — especially amid evidence that the rates of marijuana-impaired driving are on the rise.

Correction: The standard some states have established is 5 nanograms.

How marijuana legalization in Washington, Colorado and Oregon is working out so far. (Daron Taylor,Danielle Kunitz/The Washington Post)

Keith HumphreysKeith Humphreys is a Professor of Psychiatry at Stanford University and is an affiliated faculty member at Stanford Law School and the Stanford Neurosciences Institute.